Since late last week, “there have been repeated reports of airstrikes involving the use of incendiary weapons and advanced munitions such as bunker buster bombs,” the statement continues.

“The secretary-general underlines that the apparent systematic use of these types of indiscriminate weapons in densely populated areas may amount to war crimes.”

NPR’s Alice Fordham reports there is also ground fighting around the city, and forces loyal to Syrian president Bashar Assad, whose regime is backed by Russia, are gaining ground north of Aleppo, trying to cut off supply lines to rebel-held parts of the city.

Alice reports:

“People in the rebel-held part of Aleppo say that they fear a complete siege, a tactic used across Syria by several sides in the multi-factional war, although the vast majority of those suffering are besieged by the regime in opposition-held areas.

“Witnesses [tell] NPR much bigger missiles are hitting them than before, and that they believe white phosphorus is being used as a weapon, which is illegal.

…

“People in the opposition-held part of the city of Aleppo speak of an unprecedented bombardment, collapsed buildings [and] women and children among dozens killed. Water has been cut off to the entirety of the city, including the government-held area.”

“I want to make this clear, what is happening in Aleppo today is unacceptable. It is beyond the pale.

“If people are serious about wanting a peaceful outcome to this war, then they should cease and desist bombing innocent women and children, cease cutting off water and laying siege in medieval terms to an entire community, and work with the international community in order to be able to bring peace to people who are starving – literally starving, but also starving for the possibilities of a future without chlorine dropping on them out of the sky, barrel bombs, and indiscriminate bombing.”

Referring to the recent bombing of a U.N. aid convoy on its way to a rebel-controlled area west of Aleppo city, Kerry called on Russia to, “set an example, not a precedent, an unacceptable precedent, I might add, for the entire world.” While it’s not clear who carried out the attack that hit the convoy, NPR’s Alice Fordham reported that “American officials say the perpetrators could only be the Syrian air force or its Russian allies.”

The U.K.-based watchdog group the Syrian Observatory posted a statement Saturday saying at least 45 civilians had been killed in airstrikes in the eastern part of Aleppo, and said the death toll would likely rise because more people were trapped in the rubble or critically injured.